The present invention is directed, in general, to wireless and wireline receivers and, more specifically, to a system and method for cancelling precursor intersymbol interference (ISI) in a receiver.
The rapid advance of digital technology has created a great demand for, and corresponding advances in, wireless and wireline technology for communicating voice and data traffic. Much of this traffic is carried by the public switched telephone network over fiber optic cable and copper wire. Computers and other data equipment communicate over the Internet and a variety of proprietary local area networks (LANs) and wide area networks (WANs). Increasingly, various types of digital subscriber line (DSL) service or cable modem service are bringing broadband data into homes and offices. Many third generation cellular telephones and wireless PDA devices are also equipped to handle broadband data traffic and Internet capable.
However, even the most modern of wireless and wireline data communication equipment still must contend with the age-old problems inherent in transmitting data through a channel from a transmitter to a receiver. Data is often transmitted as a series of pulses (or symbols) through a wire or the atmosphere. The data symbols may become distorted due to intersymbol interference (ISI), which is an overlap of adjacently transmitted symbols. In a wireless network, ISI may be caused by reflections of the transmitted symbols off natural objects (e.g., tress, hills) and man-made objects (e.g., buildings, brides) in the environment. The reflections cause multiple time-delayed, partially overlapping copies (echoes) of the same signal to arrive at the receiver. ISI also may occur in a non-linear, bandwidth limited channel if the symbol transmission rate is comparable to or exceeds the channel bandwidth, W.
Receivers frequently use a well-known technique, adaptive decision-feedback equalization, to minimize the effects of ISI. An adaptive decision-feedback equalizer (DFE) consists of a feedforward (or forward) filter, a feedback filter, and a decision circuit that decides or detects the value of each symbol in the received signal. The input to the forward filter is the received distorted sequence of data symbols. The input to the feedback filter is the sequence of previously decided (detected) symbols at the output of the decision circuit. The feedback filter removes from the symbol presently being estimated that portion of the ISI that is caused by previously detected symbols.
There are limitations, however, to the performance of decision feedback equalizers. Even under the best of circumstances, a DFE occasionally makes an incorrect decision regarding the value of a received data symbol. The incorrect estimate is then propagated back to the feedback filter, thereby affecting decisions regarding subsequent symbols. Furthermore, a DFE almost always does not perform detection on the first copy of a symbol as it is received. Because of the performance of the channel, symbol reflections may combine in such a way that the peak power of the transmitted symbol occurs after the first echo of the symbol enters the DFE. Thus, some reflections of a symbol (postcursors) are received by the DFE after a symbol is detected, but other reflections of a symbol (precursors) are received by the DFE before the symbol is due to be detected. A conventional DFE is unable to compensate for precursor ISI in the detection of the present symbol because of the causal nature of the feedback filter.
For example, in a sequence of ten symbols, the DFE may be working on detecting (deciding) the fifth symbol. However, precursor ISI from the sixth and seventh symbols and post-cursor ISI of the third and fourth symbols may contribute to distortion of the fifth symbol. Since the third and fourth symbols have already been decided by the decision circuit, the feedback loop can be used to remove the postcursor ISI. However, since the sixth symbol has not been detected yet, the feedback filter does nothing to remove the precursor ISI.
There is therefore a need in the art for improved receivers and transmitters for use in communication networks. In particular, there is a need in the art for improved decision feedback equalizers that have a lower detected symbol error rate. More particularly, there is a need for receivers containing decision feedback equalizers (DFEs) that are capable of at least partially minimizing precursor ISI due to symbols that have not yet been detected. Moreover, there is a need for improved transmitters and A data networks that are capable of maximizing the performance of receivers that contain decision feedback equalizers capable of reducing precursor ISI.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an apparatus for reducing a precursor ISI signal for use in a receiver capable of receiving from a transmission channel an incoming stream of symbols distorted by intersymbol interference (ISI). In an advantageous embodiment of the present invention, the apparatus comprises: 1) a first forward filter capable of receiving the incoming stream of distorted symbols and generating a first equalized output signal (Yxe2x80x2k); 2) a first decision feedback equalizer (DFE) stage capable of receiving the Yxe2x80x2k signal and generating a first decided symbol sequence (Ŝkxe2x88x92d), wherein the first DFE stage generates from Ŝkxe2x88x92d a first postcursor cancellation signal capable of reducing postcursor ISI in the Yxe2x80x2k signal and generates from the Yxe2x80x2k signal a first symbol estimate signal (vk) in which postcursor ISI is at least partially reduced; and 3) a second decision feedback equalizer (DFE) stage capable of receiving the Yxe2x80x2k signal and generating a second decided symbol sequence (Ŝkxe2x88x922d), wherein the second DFE stage comprises a soft symbol estimator capable of receiving the vk signal and generating a soft decision sequence (Sxe2x80x2kxe2x88x92d) comprising decided symbols and zero output signals, and wherein the second DFE stage generates from Sxe2x80x2kxe2x88x92d a precursor cancellation signal capable of reducing precursor ISI in the Yxe2x80x2k signal.
According to one embodiment of the present invention, the first DFE stage further comprises a first feedback filter capable of receiving Ŝkxe2x88x92d and generating therefrom the first postcursor cancellation signal.
According to another embodiment of the present invention, the first DFE stage further comprises a first summer capable of adding the Yxe2x80x2k signal and the first postcursor cancellation signal to thereby generate the vk signal.
According to still another embodiment of the present invention, the first DFE stage further comprises a first symbol estimator capable of receiving the vk signal and generating the first decided symbol sequence, Ŝkxe2x88x92d;.
According to yet another embodiment of the present invention, the second DFE stage further comprises a delay circuit capable of receiving and delaying the Yxe2x80x2k signal and a second feedback filter capable of receiving Ŝkxe2x88x922d and generating therefrom a second postcursor cancellation signal capable of reducing postcursor ISI in the delayed Yxe2x80x2k signal.
According to a further embodiment of the present invention, the second DFE stage further comprises a third feedback filter capable of receiving Sxe2x80x2kxe2x88x92d and generating therefrom a precursor cancellation signal capable of reducing precursor ISI in the delayed Yxe2x80x2k signal.
According to a still further embodiment of the present invention, the second DFE stage further comprises a summer capable of adding the delayed Yxe2x80x2k signal, the precursor cancellation signal, and the second postcursor cancellation signal to thereby generate a second symbol estimate signal (vxe2x80x2k) in which precursor ISI is at least partially reduced.
According to a yet further embodiment of the present invention, the second DFE stage further comprises a second symbol estimator capable of receiving the vxe2x80x2k signal and generating the second decided symbol sequence, Ŝkxe2x88x922d.
In another embodiment of the present invention, the soft symbol estimator generates a known symbol if a value of the vk signal exceeds a minimum threshold and generates a zero output signal if the value of the vk signal does not exceed the minimum threshold.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.